This disclosure relates generally to the field of pulsed neutron well logging instruments. More specifically, the disclosure relates to methods for obtaining values of compensated neutron capture cross section (sigma) from such instruments.
Formation neutron capture cross section (sigma) measurement based on measurements from a pulsed neutron well logging instrument having capture gamma ray detectors disposed at axially spaced apart locations from a pulsed neutron generator (PNG) has been used in the oil and gas exploration and production industry for several decades. Such measurements may be referred to as “thermal neutron die-away measurements”, and they are related to the determination of the macroscopic thermal neutron capture cross section of the formation. The decay of the thermal neutron population after a “burst” of high energy (in the one million electron volt and above energy range) neutrons from the PNG is close to exponential in an ideal situation (for example, a homogeneous uniform medium surrounding the measuring instrument), while the actual neutron population decay cannot be represented by an analytic formula. An apparent decay constant (based on either a curve fitting or moments method) is not always representative of the “intrinsic” formation “decay constant”, which is inversely proportional to the macroscopic thermal neutron capture cross section (sigma) of the formation surrounding a wellbore from within which measurements are usually made. Wellbore decay contamination of the measured neutron population and thermal neutron diffusion affect the apparent decay constant. Under some conditions, for example, salty (i.e., high chloride content) fluid in the wellbore and values of sigma of the formation much smaller than sigma of the wellbore fluid, at least two decay constants can be observed in the measured neutron population with respect to time from the end of a neutron burst, and the decay of the thermal neutron population is very close to a dual exponential decay. However, such dual exponential decay is not always clearly identifiable in the neutron population data. An accurate and precise formation sigma determination from pulsed neutron capture measurements under substantially all conditions and with minimal correction for the wellbore environment is a challenging technical problem.